The present invention, in some embodiments thereof, relates to the design, construction and uses of molecular nano-motors and, more particularly, but not exclusively, to molecular rotary motors based on a rotaxane architecture which includes a cucurbit[n]uril and an uncharged guest molecule having low or null affinity therebetween.
Nanotechnology is a broad term used to describe the field of applied science and technology dealing with the control of matter on the atomic and molecular scale, typically from 1 nm to 100 nm, and the fabrication of devices in this order of magnitude. Nanotechnology stems from the fields of supramolecular chemistry, biology, applied physics, materials science, mechanical and electrical engineering, colloidal science, device physics, and others. The shift from standard mechanics to nano-scale mechanics is non-trivial since the physical, chemical, and biological properties of materials may differ fundamentally at the nanoscale level from the bulk properties of the materials, leading to unexpected results because of variations on the quantum mechanical properties of atomic interactions.
Molecular motors, as used in the context of the present invention, are nano-scaled structures that are likely to prove especially valuable in the emerging field of nanotechnology. The overall significance of nano-scaled motors to nanotechnology is comparable to the impact of the engine in modern society. The ability to harness and utilize, to both construct and deconstruct, these motors has the potential to expand and revolutionize the field of nanotechnology.
Biological proteinous molecular motors precedents [1], such as bacterial flagellar motor [2], F1-ATPase (ATP synthase)[3-5], kinesin, myosin [6] and helicase [7], which interconvert chemical energy and coordinated mechanical motion, transport and manipulate cellular components.
Synthetic molecular motors, which are not based on proteins, have been theorized and hypothesized in the past decades [8-12]. Most of the reported efforts to synthesize molecular rotary motors, including bevel gears, propellers, a three-propeller system, and molecular turnstiles [13-27] have exploited intramolecular interactions with one molecular fragment rotating with respect to the rest of the molecule around one or two single bonds [28-33].
One of the most challenging design elements of molecular rotary motors is their requirement of a unidirectional motion. Attempts to meet this challenge have resulted in various stepwise (non-continuous), unidirectional moving devices [34, 35], including light-driven [36-38], chemically driven [8, 39, 40], and electrically driven machines [22, 24, 41, 26, 27].
A synthetic chemically driven rotary molecular motor was reported by Kelly et al. in 1999 [39], which provided a system based on a three-bladed triptycene rotor and a [4]helicene molecule, which was capable of performing a unidirectional 120° rotation. The molecular motor of Kelly et al. is an elegant example of how chemical energy can be used to induce controlled, unidirectional rotational motion, a process which resembles the consumption of ATP in organisms in order to fuel numerous processes. However, it does suffer from lack of repeatability of the sequence of events that leads to 120° rotation, and attempts to overcome this limitation have not been successful hitherto.
Fering a et al. reported in 1999 [36] the creation of a unidirectional molecular rotor. Their 360° molecular motor system consists of a bis-helicene connected by an alkene double bond displaying axial chirality and having two stereocenters, wherein one cycle of unidirectional rotation takes four reaction steps. This system was characterized by low speed due to the long reaction time needed to complete one rotation in these systems, which does not compare to rotation speeds displayed by motor proteins in biological systems. Fering a et al. continued to improve the speed of these light-driven unidirectional molecular motors and provided faster system with a fluorene lower half, exhibiting a half-life of the thermal helix inversion of 0.005 seconds [42, 43]. The Fering a principle has been incorporated into a prototype nanocar [44], based on an helicene-derived engine with an oligo-phenylene ethynylene chassis and four carborane wheels and is expected to be able to move on a solid surface with scanning tunneling microscopy monitoring, although this has not been observed to date.
An alternative approach to such machines exploits the rotaxane's mechanically-interlocked molecular architecture, which consists of a macrocyclic molecule threaded by a linear “dumbbell shaped molecule” guest molecule that is terminated by two bulky stopper moieties [45-48]. Rotaxanes can exhibit three types of motion: rotation of a wheel around an axle (or a rotator inside a stator, which depends on the frame of reference), shuttling of the wheel along the axle in a piston-like motion, and a pivoting motion, where the angle between the axle and the main axis of the host changes. The shuttling motion has been studied in great detail both experimentally and theoretically [49]. A similar motion of circumrotation in catenanes was also investigated [35]. External stimuli, such as light, thermal energy, or electrochemical energy have been used to control the motion, including the threading/unthreading motion of pseudo rotaxanes [50-54]. An external electric field has been shown to induce the rotation of a rotaxane wheel around its axle, demonstrating that rotaxanes could interconvert different types of energy and therefore may potentially be used as energy converters [41].